Video presentations are often augmented with special effects to entertain, inform or otherwise capture the interest of viewers. One method of generating special effects is known as chromakeying. By this method, an opaque backdrop is provided in the field of view of the video camera where the special effects are desired. The backdrop may comprise a curtain, screen or wall, but it must be uniformly colored, usually in a shade of blue or green. When special effects are desired to appear in the video presentation, the special effect images are electronically substituted for the color signals representing the images of the opaque backdrop. Thus, wherever a signal representing the opaque backdrop appears in the video presentation, a corresponding signal representing the special effect will be substituted therefor. In this way, the special effects will appear in the video presentation in place of the opaque backdrop.
The method of adding special effects through chromakeying has been extensively used in television broadcasting. For example, during the broadcast of a weather report, the viewer is presented with the illusion of a television personality standing in front of a weather map. This illusion is created by positioning the television personality in front of a uniformly colored opaque backdrop. The video camera generates a video signal corresponding to the color of the backdrop. The video signal generated by the camera goes to known video control apparatus which reads the signals corresponding to the opaque backdrop and substitutes therefor corresponding signals representing the weather map. Thus, the viewer sees the television personality standing in front of a weather map rather than an opaque backdrop.
As explained above, chromakeying substitutes a special-effect video signal for a corresponding opaque backdrop signal of a certain color. Therefore, care must be taken to avoid including anything in the field of view of the camera which is close to the color of the opaque background. Dressing a television personality in colors close to the color of the opaque backdrop may cause a substitution of the special effects in an undesired area, thereby destroying the special effect illusion. Blues and greens are the colors of choice for opaque backdrops because they are the colors most opposite flesh tones on the color wheel. The use of blue or green in connection with the backdrop creates a sharper image in the video presentation of such areas as a broadcaster's face and hands, and minimizes the possibility of a special effects overlap in these areas.
Although chromakeying offers the benefits of augmenting a video presentation with special effects, the use of an opaque backdrop has several drawbacks. First, the opaqueness of the backdrop creates certain problems. If chromakeying is not used to mat the opaque backdrop with special effects, the opaque backdrop will be part of the video presentation. This may not be aesthetically acceptable. Further, disclosing the existence of the opaque backdrop may destroy or seriously undermine the illusion created by the special effects. Special camera angles may be necessary to avoid including the opaque backdrop in the camera's field of view. This limits the number of available video presentation options. Where time and flexibility are at a premium, such as in unpredictable live television presentations, limiting the camera angles may affect the quality of the video presentation.
In addition, performers in a video presentation must be careful to maintain the illusion created by the special effects despite the difficulties presented by the opaqueness of the backdrop. For example, during a weather report, a weather map may be generated behind the television personality as a special effect. It appears to the viewer that the television personlaity is gesturing with respect to the weather map. However, in the studio, the television personality is gesturing with respect to the opaque backdrop. He or she must look off-camera to a teleprompter or monitor to determine whether his or her gesticulations are accurate. Even the most experienced weather forecaster may look awkward sometimes or otherwise compromise the quality of the presentation in searching for the location of the prompter or monitor off-camera.
In certain video presentations, the special effect of a "window" may be desired. For instance, graphics relating to a news story may be visible in a window positioned relative to the television personality during a news broadcast. It is not possible to limit special effects to a window when an opaque background such as a curtain, screen or wall covering a large area is used.
Another drawback of opaque backdrops is the creation of shadows. During a video presentation, a person or other object may be positioned in front of an opaque backdrop in such a manner that shadows are created on the backdrop. This may affect the backdrop so as to interfere with the substitution of the special-effects signals for the opaque backdrop signals.
Further, an opaque backdrop is usually fixed in location for the duration of the video presentation, or even permanently. Fixing the location of the opaque backdrop generally also fixes the focal length of the video camera. As with the necessity of special camera angles, fixing the location of the opaque backdrop limits the available video presentation options. Futher, fixing the focal length may limit the types of special effects presented in connection with the opaque background. Objects not part of the special effects, such as television personalities, may appear to shrink or swell in connection with changes in the special effects, especially in connection with close-up and zoom presentations.
Accordingly, there is a need in the art for a backdrop for use in connection with video apparatus which is transparent so as to minimize its interference with the video presentation, which is variable in size so as to be suitable for a variety of special effects, and which is readily movable so as to satisfy differing video presentation requirements.